#include <asio.hpp>
#include <array>
#include <iostream>
#include <memory>

using asio::ip::tcp;

namespace third_party_lib
{

// Simulation of a third party library that wants to perform read and write
// operations directly on a socket. It needs to be polled to determine whether
// it requires a read or write operation, and notified when the socket is ready
// for reading or writing.
class session
{
public:
    session(tcp::socket& socket)
        : socket_(socket)
    {
    }

    // Returns true if the third party library wants to be notified when the
    // socket is ready for reading.
    bool want_read() const
    {
        return state_ == reading;
    }

    // Notify that third party library that it should perform its read operation.
    void do_read(std::error_code& ec)
    {
        if(std::size_t len = socket_.read_some(asio::buffer(data_), ec))
        {
            write_buffer_ = asio::buffer(data_, len);
            state_ = writing;
        }
    }

    // Returns true if the third party library wants to be notified when the
    // socket is ready for writing.
    bool want_write() const
    {
        return state_ == writing;
    }

    // Notify that third party library that it should perform its write operation.
    void do_write(std::error_code& ec)
    {
        if(std::size_t len = socket_.write_some(
                                 asio::buffer(write_buffer_), ec))
        {
            write_buffer_ = write_buffer_ + len;
            state_ = asio::buffer_size(write_buffer_) > 0 ? writing : reading;
        }
    }

private:
    tcp::socket& socket_;
    enum { reading, writing } state_ = reading;
    std::array<char, 128> data_;
    asio::const_buffer write_buffer_;
};

} // namespace third_party_lib

// The glue between asio's sockets and the third party library.
class connection
    : public std::enable_shared_from_this<connection>
{
public:
    connection(tcp::socket socket)
        : socket_(std::move(socket))
    {
    }

    void start()
    {
        // Put the socket into non-blocking mode.
        socket_.non_blocking(true);

        do_operations();
    }

private:
    void do_operations()
    {
        auto self(shared_from_this());

        // Start a read operation if the third party library wants one.
        if(session_impl_.want_read() && !read_in_progress_)
        {
            read_in_progress_ = true;
            socket_.async_wait(tcp::socket::wait_read,
                               [this, self](std::error_code ec)
            {
                read_in_progress_ = false;

                // Notify third party library that it can perform a read.
                if(!ec)
                {
                    session_impl_.do_read(ec);
                }

                // The third party library successfully performed a read on the
                // socket. Start new read or write operations based on what it now
                // wants.
                if(!ec || ec == asio::error::would_block)
                {
                    do_operations();
                }

                // Otherwise, an error occurred. Closing the socket cancels any
                // outstanding asynchronous read or write operations. The
                // connection object will be destroyed automatically once those
                // outstanding operations complete.
                else
                {
                    socket_.close();
                }
            });
        }

        // Start a write operation if the third party library wants one.
        if(session_impl_.want_write() && !write_in_progress_)
        {
            write_in_progress_ = true;
            socket_.async_wait(tcp::socket::wait_write,
                               [this, self](std::error_code ec)
            {
                write_in_progress_ = false;

                // Notify third party library that it can perform a write.
                if(!ec)
                {
                    session_impl_.do_write(ec);
                }

                // The third party library successfully performed a write on the
                // socket. Start new read or write operations based on what it now
                // wants.
                if(!ec || ec == asio::error::would_block)
                {
                    do_operations();
                }

                // Otherwise, an error occurred. Closing the socket cancels any
                // outstanding asynchronous read or write operations. The
                // connection object will be destroyed automatically once those
                // outstanding operations complete.
                else
                {
                    socket_.close();
                }
            });
        }
    }

private:
    tcp::socket socket_;
    third_party_lib::session session_impl_{ socket_ };
    bool read_in_progress_ = false;
    bool write_in_progress_ = false;
};

class server
{
public:
    server(asio::io_context& io_context, unsigned short port)
        : acceptor_(io_context, { tcp::v4(), port })
    {
        do_accept();
    }

private:
    void do_accept()
    {
        acceptor_.async_accept(
            [this](std::error_code ec, tcp::socket socket)
        {
            if(!ec)
            {
                std::make_shared<connection>(std::move(socket))->start();
            }

            do_accept();
        });
    }
    tcp::acceptor acceptor_;
};

int main(int argc, char* argv[])
{
    try
    {
        if(argc != 2)
        {
            std::cerr << "Usage: third_party_lib <port>\n";
            return 1;
        }
        asio::io_context io_context;
        server s(io_context, std::atoi(argv[1]));
        io_context.run();
    }
    catch(std::exception& e)
    {
        std::cerr << "Exception: " << e.what() << "\n";
    }
    return 0;
}